Why does ABO incompatibility happen?
Last updated September 7, 2026.
**ABO incompatibility happens when antibodies recognize A or B markers on another person's red blood cells as foreign and attack them.** Your inherited blood type determines which A and B markers your red blood cells carry. If you receive incompatible donor red blood cells, your antibodies can destroy them, which is why your care team checks compatibility before a transfusion. During pregnancy, some antibodies can cross the placenta and break down your baby's red blood cells if your blood types are incompatible. A pregnancy mismatch does not always cause illness, but it can cause newborn jaundice or anemia. Keep your prenatal testing and newborn follow-up appointments so your clinician can identify problems early.
What to do
- **Complete prenatal testing:** Have the blood type and antibody tests your prenatal clinician recommends.
- **Keep newborn visits:** Attend your baby's scheduled checks for jaundice, feeding, and weight.
- **Report yellowing promptly:** Contact your baby's clinician promptly if you notice yellow skin or eyes, especially during the first day after birth.
- **Confirm your identity:** Before a transfusion, help staff verify your name and date of birth against your wristband.
- **Speak up during transfusions:** Tell your nurse immediately if you develop chills, fever, back pain, or trouble breathing.
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